1. Field of the Invention
The present invention relates to an image sensor.
2. Description of the Background Art
A CCD (charge-coupled device) image sensor comprising a multiplier (charge increaser) multiplying (increasing the quantity of) electrons (charges) is known in general.
In such a conventional multiplier CCD image sensor, a gate oxide film 502 is formed on the surface of a silicon substrate 501, as shown in FIG. 25. Four gate electrodes 503 to 506 are formed on a prescribed region of the upper surface of the gate oxide film 502 at prescribed intervals. The gate electrodes 503 to 506 are supplied with four-phase clock signals φ11 to φ14 respectively.
A pixel separation barrier, a temporary storage well, a charge transfer barrier and a charge accumulation well are formed in portions of a transfer channel 507 located under the gate electrodes 503 to 506 respectively. The pixel separation barrier has a function of dividing the temporary storage well from a charge accumulation well of an adjacent pixel while transferring electrons from the adjacent charge accumulation well to the temporary storage well. The temporary storage well temporarily has a function of temporarily storing the electrons transferred thereto. The charge transfer barrier has a function of dividing the temporary storage well and the charge accumulation well from each other while transferring the electrons stored in the temporary storage well to the charge accumulation well.
The charge accumulation well has a function of accumulating the electrons transferred from the temporary storage well, and also functions as a multiplier for multiplying the electrons by collisional ionization caused by an electric field. In other words, a high-field region 508 regulated to a high potential is formed on the interface between the charge transfer barrier and the charge accumulation well, thereby supplying energy to electrons transferred thereto. The electrons supplied with energy collide with atoms in a silicon substrate 501 during transition through the high-field region 508, thereby forming electrons and holes. Among the generated electrons and holes, only the electrons are accumulated in the charge accumulation well due to the electric field in the high-field region 508. Thus, the electrons are multiplied. The conventional multiplier CCD image sensor multiplies the electrons in a process of transferring electrons generated by a photodiode of a photoreceiving region.
The multiplying operation of the conventional multiplier CCD image sensor is now described with reference to FIG. 25.
First, a high-level clock signal φ11 is supplied to turn on the gate electrode 503, and the gate electrode 506 of the adjacent pixel is thereafter turned off. Thus, electrons stored in the charge accumulation well of the adjacent pixel are transferred to the pixel separation barrier.
Then, a high-level clock signal φ12 is supplied to the gate electrode 504 for turning on the gate electrode 504, and a low-level clock signal φ11 is thereafter supplied to the gate electrode 503 for turning off the gate electrode 503. Thus, the electrons transferred to the pixel separation barrier are transferred to the temporary storage well.
Then, a high-level clock signal φ14 is supplied to the gate electrode 506 for turning on the gate electrode 506. Thus, a high voltage is applied to the gate electrode 506, for forming the high-field region 508 on the interface between the charge transfer barrier and the charge accumulation well. Thereafter a low-level clock signal φ12 is supplied to the gate electrode 504 for turning off the gate electrode 504 while keeping the gate electrode 506 in the ON-state, thereby transferring the electrons stored in the temporary storage well to the charge accumulation well over the charge transfer barrier Thus, the transferred electrons are multiplied by collisional ionization caused by a high electric field, and the multiplied electrons are stored in the charge accumulation well. The gate electrode 505 is supplied with clock signal φ13 of a constant voltage, and the charge transfer barrier is regulated to a prescribed potential and kept constant.